1. Field of the Invention
5-fluoro-1H-pyrazoles, in particular 5-Fluoro-1-methyl-3-pentafluoroethyl-4-trifluoromethyl-1H-pyrazole, are important building blocks for the preparation of crop protection chemicals, as those described in WO 2010051926.
2. Description of Related Art
It is known that 5-fluoro-1-methyl-3-pentafluoroethyl-4-trifluoromethyl-1H-pyrazole can be prepared by the treatment of the dimer of hexafluoropropene with water free N,N-dimethylhydrazine in diethyl ether at −50° C. followed by heating of the intermediate at 120° C., I. L. Knunyants et al. Izv. Akad. Nauk SSSR, (1990) 2583-2589.

However, this two step transformation requires low temperatures for the first step and results in the formation of CH3F during the thermal elimination in the second step, making this process expensive, environmentally unfriendly, and particularly difficult for industrialization
Starting from perfluoro-2-methyl-2-penten and phenylhydrazine, in the presence of triethylamine at −50° C. 1-Phenylpyrazole was prepared in 90% yield (SU 1456419). Furin et al. J. Fluor. Chem. 98(1999) 29 reported that the areaction of perfluoro-2-methyl-2-penten with phenylhydrazine in CH3CN gave a mixture of isomeric pyrazoles 3 and 4 in a ratio 4:1.

The utilization of the commercially available and cheap monoalkylhydrazines (especially in the form of their water solutions) for the regioselective synthesis of the said pyrazoles is not known to the person of ordinary skill in the art.